I tried to create a file in my profile folder: chrome/userChrome.css
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
#back-button, #forward-button {
display: none!important;
}
I also tried
@-moz-document url(chrome://browser/content/browser.xul) {
#back-button, #forward-button { display: none!important; }
}
and
#nav-bar #back-button, #nav-bar #forward-button {
display: none !important;
}
But none of this has any effect (even after firefox restart). I guess, this is the old way of achieving this (in firefox 58)
Note: if you are on this path, note that you won't have any more access to the back list dropdown